When have you changed the hydraulic filter?
Gravity can't force the oil being returned to the tank through a partly clogged filter very fast.
Other hydraulics (if you have them) might operate with double acting cylinders which actually power in......and out... under hydraulic pressure, so those things don't show a lot of slow down due to a clogged filter. But even though double acting cylinders don't slow down that much, there is still reason to be concerned because where a gravity down single acting cylinder might not unseat the bypass in the filter.................double acting cylinders certainly will and that means the filter is just circulating unfiltered oil when it goes into bypass.